Present-day direct access storage units usually employ a magnetizable recording medium that is rotated relative to an essentially stationary magnetic transducer. The transducer, or magnetic head, typically comprises a magnetizable core providing a path for the magnetic flux generated by currents flowing in a coil wound on said core and creating a magnetic field across the gap of the core. The gap faces the surface of the recording medium at a preferably very small but constant distance. As the magnetic field varies, these variations are "written" into the recording medium. Conversely, for "reading", as magnetic transitions stored on the recording medium travel past the pole tips at the gap of the magnetic core, flux is induced in the windings of the coil and can be read out by suitable electronic circuitry.
One commercially available direct access storage unit has an areal density of about 1.2 million bits per square centimeter which is the equivalent of a storage area of 80 .mu.m.sup.2 per bit. This density is achieved with a gap of about 1 .mu.m width and a flying height of the transducer over the recording surface of less than 250 nm created by a pressurized air bearing generated automatically as the recording medium rotates.
A great step forward toward reducing the distance between the transducer and recording medium is disclosed in U.S. Ser. No. 909,799, filed Sept. 22, 1986, wherein the flying height is controlled by a feedback loop arrangement which uses the tunneling current flowing across the gap between transducer and recording medium. This permits the gap to be as small as several tens of nanometers and, accordingly, an increase in storage density at least by a factor of 10 with respect to those storage units relying on an air bearing for the transducer flying height regulation.
In addition to signal amplitude, domain size and other factors, the recording density is dependent on the size of the transducer, namely the size of the gap of the magnetic head. The small gap width of 1 .mu.m is achieved with a thin film head. Photolithographic techniques enable controlling the dimensions of the head to within a few percent and permit placement of the winding with micrometer accuracy. However, it is difficult to reduce the gap width below 1 .mu.m with conventional techniques. Alternative solutions are therefore necessary to permit a further reduction in the bit size and increase in storage capacity per unit area.